Computing networks can include multiple network devices such as routers, switches, hubs, servers, desktop PCs, laptops, and workstations, among other peripheral devices, e.g., printers, facsimile devices, and scanners, networked together across a local area network (LAN) and/or wide area network (WAN).
In addition to physical connections, networks often form virtual, e.g., logical connections. A virtual LAN (VLAN) is a logical subgroup within a LAN that is created via software rather than manually moving cables in the wiring closet. It combines user stations and network devices into a single unit regardless of the physical LAN segment to which they are attached and allows traffic to flow more efficiently within populations of mutual interest.
VLANs are implemented in port switching hubs and LAN switches and generally offer proprietary solutions. VLANs reduce the time it takes to implement connection moves, additions and changes. VLANs function at layer 2 of the open system interconnection (OSI) protocol stack. The OSI protocol stack is one example of the set of protocols used in a communications network. A protocol stack is a prescribed hierarchy of software layers, starting from the application layer at the top (the source of the data being sent) to the physical layer at the bottom (transmitting the bits on the wire). The stack resides in each client and server, and the layered approach lets different protocols be swapped in and out to accommodate different network architectures. Layer 2 is the data link layer. Layer 2 is responsible for node to node validity and integrity of the transmission. The transmitted bits are divided into frames; for example, an Ethernet, Token Ring or FDDI frame in local area networks (LANs).
VLAN tagging is used to divide physical ports (P) on a switching device into a set of virtual groups (V), where each group is called a VLAN. In each VLAN, a port is either tagged or untagged. Taggedness of a port determines whether additional information needs to be prepended to the port before sending data out of that port. A single port may be a member of multiple VLANs, but may only be marked untagged in one VLAN.
One issue with VLAN tagging is when any discrepancies arise between the configuration of the switch and the physical orientation of links, e.g., physical (Layer 1) connections, to ports on other switches. If, for example, a port P1 on switch S1 is configured to be tagged on VLANs V1 and V2 and untagged on VLAN V3, then this configuration is only useful when P1 is properly linked to another switch (S2) with V1, V2, and V3 configured identically. If one of the following two instances arise, then the network connection may be faulty: (1) the link is not plugged in to port P1, but is mistakenly plugged into some other port, e.g., P2 on S1; (2) the link is properly plugged in, but the configuration of S2 does not match the configuration of S1. In these two cases, it is up to the network administrator to detect the problem, locate the root cause, and provide a remedy.